Redox potential correlation with kinetics

Redox potentials of pentacyanoferrates are often determined in association with kinetic and stability constant determinations. They are also available for 4-methyl- and 4-amino-pyri-dine pentacyanoferrates, and for [Fe(CS )5(2,6-dimethylpyrazine)] in acetonitrile-water mixtures.Oxidation potentials of [Fe(CN)5L] complexes correlate with the electron-withdrawing or -releasing properties of the ligands... 

As noted earlier, the polarographic half-wave potential Estandard redox potential of the couple because EVl is governed by kinetic as well as thermodynamic parameters, so that attempts to correlate EVl for irreversible couples with structural features of the complex have been very difficult to obtain. One moderately successful correlation, however, was obtained by Vlcek13 and by Crow14 for a fairly extensive series of complexes of Co111, Crm and Rhm. It was suggested that before reduction of the metal complex... 

The kinetics of oxidation and reduction of [4Fe-4S] proteins by transition metal complexes and by other electron-transfer proteins have been studied. These reactions do not correlate with their redox potentials.782 The charge on the cluster is distributed on the surface of HiPIP through the hydrogen bond network, and so affects the electrostatic interaction between protein and redox agents such as ferricyanide, Co111 and Mnin complexes.782 783 In some cases, limiting kinetics were observed, showing the presence of association prior to electron transfer.783... 

The possibility of predicting thermodynamic properties of redox couples and solutes in different solvents is very important. It should be very useful to develop procedures of transferring thermodynamic data such as redox potentials from solvent to solvent. In fact, the correlation found between kinetic and thermodynamic parameters of reactions in solutions, and solvent parameters such as DN, AN, dielectric constant, etc., indicates that it may be quite feasible to draw empirical formulas which predict, for instance, redox potentials in some solvents, based on well-established data obtained experimentally with other solvents. Thus, it may be possible to define transfer parameters (AG , AH , ASf, etc.) reflecting the difference between aqueous and polar aprotic solutions in the thermodynamic properties of solutes. 

A number of observations [6, 83, 84] have indicated that the heme domain of Fcyt h is mobile relative to the flavin domain, and it has been suggested that this conformational mobility may influence the intramolecular ET kinetics [80, 81, 85]. Inasmuch as the pyruvate effects on intramolecular ET do not correlate with the changes in redox potentials, they may provide an example of conformational gating [86, 87]. This remains to be established (see [82] for further discussion). In this context, it is interesting that in the Fcyt h crystal, bound pyruvate apparently acts to increase the mobility of the heme domain to the point where it is no longer visible in the X-ray electron density map [6]. 

A second fluorescence increase titrates with an apparent midpoint potential of — 0.25 to —0.30 V [90,94] (at these potentials the acceptor has been designated Ql, as opposed to the high potential Qh)- Also, the of is pH dependent, suggesting that it could be another plastosemiquinone/plastoquinone couple. The presence of Ql and could be related to the kinetic pattern of the fluorescence rise, which is clearly biphasic [95]. When the rate, and not the extent, of the fluorescence increase is studied as a function of the redox potential, the faster component of the rise correlates with Ql and the slower with Q [96,97], Ql seems therefore to be the most efficient quencher on the other hand Q is usually the more prominent component of the fluorescence rise and correlates with the electro-chromic signal C-550 linked to the reduction of A 2 [98]. 

Figure 11.15 (from Stone, 1987) illustrates that the rate of oxidation of various substituted phenols by Mn02 can be correlated with the half-wave potentials, of these phenols. The half-wave potentials, measure the tendency of the anode to oxidize the phenols. The half-wave potential corresponds in first approximation to the redox potential of the phenol and its one-electron oxidation product. Figure 11.15 implies that the thermodynamic tendency of an electrode to oxidize a certain phenol relates to the kinetic tendency of Mn02 to oxidize this phenol. 

The reactivity of short-lived bimetallic clusters has also been studied by the kinetics method. Under conditions when a transient alloyed cluster of Ag-Au was formed,reactivity with the electron donor MV+ was probed and compared with that of monometallic Ag clusters previously observed. Just after the pulse a mixed solution of Ag and Au cyanides is partially reduced into atoms Ag and Au , while is partially reduced to the redox probe MV+. It is observed that in the first 20 ms the kinetics, at 400 nm, of cluster growth are the same as in the absence of the probe. Thus the coalescence of atoms to form an alloyed small cluster is, at first, not affected. The mechanism should be the same as in Eqs. (20)-(23). After this period, however, the decay of MV" at 700 nm starts in correlation with the increase of the cluster absorbance which results from electron transfer (Fig. 12). When the bimetallic cluster formed reaches the critical size where its potential becomes slightly higher than °(MV +/MV+ ), it acts as a nucleus that initiates a catalyzed growth fed alternately by electron transfer from the donor and the adsorption of excess Ag or Au ions. For i +J > ny. 

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